The overarching aim of the present NRSA proposal is to better understand vulnerability processes for panic disorder (PD) by empirically delineating the main and interactive effects of anxiety sensitivity (AS;fear of anxiety) and nonclinical unexpected panic attacks. This proposal integrates theory and research in a novel manner to test the hypothesis that AS interacts with a (recent) history of unexpected panic attacks in a nonclinical population to account for PD-relevant responding to a challenge involving the administration of 10% carbon dioxide-enriched air (CO2). The specific hypotheses of the present study are to test whether AS interacts with a history of nonclinical unexpected panic attacks in the prediction of responding to a CO2 procedure in terms of: (1) level of post-challenge anxiety focused on bodily sensations and intensity of endorsed panic attack symptoms;(2) level of expired CO2 and skin conductance response;(3) greater levels of behavioral avoidance of a future challenge provocation;and (4) delayed recovery from the challenge in terms of self-reported anxiety and skin conductance reactivity. Understanding the key risk factors (e.g. AS) that interplay with a (nonclinical) panic attack history to confer vulnerability for PD is necessary for understanding which individuals with this history will actually go on to develop PD psychopathology. This work is clinically significant in that it not only addresses a critical gap in empirical knowledge in panic psychopathology research and theory, but also serves as a model for conducting similar types of laboratory-based multi-risk factor tests for other anxiety disorders in the future.